Multilayered optical information-recording media and process for manufacture thereof

ABSTRACT

This invention provides an optical disk effective in both precision and productivity, and to a process for the manufacture thereof. The optical disk allows reading of the information recorded on each information-recording surface of the two or more information-recording surfaces by varying the focal position of a playback laser beam incident on and passing through said light transmitting substrate; and the optical disk is provided with a transparent layer that is laminated onto the light transmitting substrate and is constituted of a thermoplastic resin sheet of uniform thickness and the transparent layer has an information-recording surface other than the surface of said light transmitting substrate formed thereon.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a mutilayered optical information-recordingmedium (hereafter, optical disk) that has two or moreinformation-recording surfaces in the thickness direction of a lighttransmitting substrate and a process for the manufacture thereof.

2. Description of the Prior Art

Optical disks which record optically readable information and allowreading of the recorded information using a laser beam spot haveheretofore been available. Optical disks including compact disks (CDs)and CD-ROMs have become widespread in use, particularly over the recentyears.

CD-ROMs recently have come to be used not only in computers but also inmultimedia game CD-ROMs and are increasingly replacing magnetic disks(floppy disks) and ROM cartridges both in computer and gameapplications. Furthermore, a high density CD version called the DVD(digital videodisk) is about to enter the field of movies andmultimedia.

Recently proposals have been made of multilayered optical disks thatenable the recording of massive quantities of information. In contrastto the conventional CD that has a single-layer information-recordingsurface in which information signals are recorded only on one surfacethereof on a substrate, the mutilayered optical disks are structured tohave a multiple number of information-recording surfaces in thethickness direction of the substrate.

Referring to FIG. 1, the aforementioned multilayered optical disk is nowdescribed below.

FIG. 1 is a drawing illustrating the structure of a conventionalmultilayered optical disk. FIG. 1 shows a partial cross-sectional viewof the optical disk in its tracking direction.

As illustrated in FIG. 1, the conventional multilayered optical disk 101has a first reflecting layer 105, a transparent layer 106, a secondreflecting layer 107, and a protective layer 108 laminated together inthis sequence onto a light transmitting substrate 104.

Crenulated pits 104A corresponding to information are generated on thesubstrate 104, and pits 104B corresponding to information are generatedon the above transparent layer 106. In other words, the optical disk 101has two information-recording surfaces in the thickness direction of thesubstrate 104, where the surface on which pits 104A are formed on thelight transmitting substrate 104 is the first information-recordingsurface 102, and the surface on which pits 104B are formed on the lighttransmitting substrate 106 is the second information-recording surface103.

The first reflecting layer 105 which is formed between the firstinformation-recording surface 102 and the second information-recordingsurface 103 is made of a material with a certain degree of lighttransmittance to enable light to be incident on the secondinformation-recording surface 103 or to be reflected.

If three or more information-recording surfaces are to be formed, asecond transparent layer is formed on the second reflecting layer 107 onthe above transparent layer 106, followed by forming the pits on thesecond transparent layer, generating a third recording surface, and asimilar procedure is used to form a fourth information-recording surfaceand beyond.

Incident playback laser beam for reading information recorded on each ofthe above information-recording surfaces (102, 103) is directed throughthe underside of the above substrate 104. The playback light beam pickupis equipped to accurately recognize the spacing between adjacentinformation-recording surfaces, which may be as narrow as several tensof microns (μm), and to be focused onto the desiredinformation-recording surface, so as to read out the information presenton each information-recording surface.

As described above, the multilayered optical disk which is structured tohave two or more information-recording surfaces in the thicknessdirection of substrate 104 can record a greater amount of informationthan can a conventional single layer structured optical disk.

Referring to FIG. 2, a conventional process for the manufacture ofoptical disk 101 having the above construction is described as follows.

First, as illustrated in FIG. 2(A), a stamper (not illustrated) with apattern negative to pits 104A on the first information-recording surfaceis used to mold, for example, by injection molding, an optical disksubstrate 104 having pits 104A on its surface.

Then, as illustrated in FIG. 2(B), a first reflecting layer 105 isformed on the surface of substrate 104 having pits 104A formed thereonby a film-forming method such as sputtering, vacuum vapor depositing,spin coating or the like. It should be recalled that as mentioned above,the first reflecting layer 105 is a reflecting film that has a certaindegree of light transmittance.

Then, a second information-recording surface is formed on top of theabove first reflecting layer 105. The second information-recordingsurface is formed by the process well-known in the art as “the 2Pprocess.”

That is, as illustrated in FIG. 2(c), a stamper 141 with crenulated 141Apattern negative to pits 104B of the second information-recordingsurface is used, a UV-curable resin 142 is deposited as a droplet ontothe first reflecting layer 105 of the above substrate 104, and then thestamper 141 is pressed at its signal-bearing surface against thesubstrate 104 having the UV-curable resin 142 deposited thereon so as tospread the UV-curable resin 142 to a uniform thickness.

Incidentally a nozzle (not illustrated) is used to suction off any resinthat has overflown from the outer peripheral of substrate 104 as thestamper 141 is pressed.

Then, when the above UV-curable resin 142 is cured by irradiating UVfrom the substrate 104 side, followed by peeling off the stamper 141, anoptical disk substrate is obtained on which is generated pits 104B thatprovide a second information-recording surface 103, as illustrated inFIG. 2(D).

Lastly, a second reflecting layer 107 is formed as a film of aluminum,gold, or the like, by sputtering or by vacuum film formation such asvacuum vapor deposition or the like, followed by forming a protectivefilm 108 onto the second reflecting layer 107 and print a label (notillustrated) on the protective layer 108, thereby completing theformation of a multilayered optical disk as illustrated in FIG. 2 (E).

It should be noted that the playback principle imposed on a multilayeredoptical disk as described above requires that the spacing betweenadjacent information-recording surfaces be under strict control. Forexample, a 40 μm thick film spacing between adjacentinformation-recording surfaces allows a variation in thickness of onlyabout ±2 μm.

However, “the 2P process” mentioned above even with improved precisioncould achieve at best a level of ±5 μm. In addition, “the 2P process” isinferior in productivity relative to that of injection molding,requiring as long as 2 minutes per surface for molding a second andsubsequent information-recording surfaces.

It may be possible to increase in practice the molding rate of “the 2Pprocess”, but that would lead to problems such as reduced film thicknessprecision, trapping of air bubbles, unsatisfactory suction-removal ofthe resin overflow, and the like.

That is, manufacture of the above multilayered optical disk by aconventional manufacturing process would be plagued with problems interms of both precision and productivity.

BRIEF SUMMARY OF THE INVENTION

1. Object of the Invention

This invention relates to a multilayered optical disk with two or moreinformation-recording surfaces in the thickness direction of a lighttransmitting substrate, and aims to provide a multilayered optical diskeffective in terms of both precision and productivity and a process forthe manufacture thereof.

2. Brief Summary

The first aspect of this invention provides an opticalinformation-recording medium that has at least two or moreinformation-recording surfaces in the thickness direction of a lighttransmitting substrate and that allows reading of the informationrecorded on each information-recording surface of the two or moreinformation-recording surfaces by varying the focal position of aplayback laser beam incident on and passing through said lighttransmitting substrate comprising:

-   -   a transparent layer that is laminated onto said light        transmitting substrate and is constituted of a thermoplastic        resin sheet of uniform thickness; wherein an        information-recording surface other than the surface of said        light transmitting substrate is formed on said transparent        layer.

Also, the second aspect of this invention provides a process for themanufacture of an optical information-recording medium that has at leasttwo or more information-recording surfaces in the thickness direction ofa light transmitting substrate and that allows reading of theinformation recorded on each information-recording surface of the two ormore information-recording surfaces by varying the focal position of aplayback laser beam incident on and passing through said lighttransmitting substrate comprising:

-   -   a first step in which a thermoplastic resin sheet of uniform        thickness is positioned between said light transmitting        substrate and a stamper having a crenulated pattern formed        thereon corresponding to pits or guide grooves; and    -   a second step in which said light transmitting substrate and        said stamper are heated, followed by pressing the light        transmitting substrate and stamper against said thermoplastic        sheet, thereby transferring the crenulated pattern formed on        said stamper to the surface of said thermoplastic resin sheet        and forming an information-recording surface on a surface other        than that of said light transmitting substrate.

The above and other related objects and features of the invention willbe apparent from a reading of the following description of thedisclosure found in the accompanying drawings and the novelty thereofpointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing illustrating a conventional multilayered opticaldisk;

FIG. 2 is a drawing illustrating an embodiment of a process for themanufacture of a conventional multilayered optical disk;

FIG. 3 is a drawing illustrating the structure of an optical disk of afirst example of this invention;

FIG. 4 is a drawing illustrating an embodiment of a process for themanufacture of the optical disk of this invention which is shown in FIG.1;

FIG. 5 is a schematic view for the essential parts of a molding devicefor the optical disk of this invention which is shown in FIG. 1;

FIG. 6 is a drawing illustrating the structure of an optical disk of asecond example of this invention; and

FIG. 7 is a drawing illustrating another embodiment of a process for themanufacture of the optical disk of this invention which is shown in FIG.1.

DETAILED DESCRIPTION OF THE INVENTION

Examples of this invention are described below referring to the attacheddrawings.

EXAMPLE 1 FOR OPTICAL DISK

First, the structure of an optical disk of this invention is describedusing FIG. 3. FIG. 3 illustrates part of the cross-sectional view of anoptical disk in its tracking direction. For simplification, themultilayered optical disk described below will be one with twoinformation-recording surfaces.

As shown in FIG. 3, the optical disk 1 of this Example has a firstreflecting layer 5, a transparent layer 6, a second reflecting layer 7,and a protective layer 8 laminated together in this sequence on a lighttransmitting substrate 4.

On substrate 4 are formed crenulated pits 4A corresponding toinformation; on the above transparent layer are formed crenulated pits4B corresponding to information. That is, the optical disk 1 has twoinformation-recording surfaces in the thickness direction of thesubstrate 4, wherein the surface on which the pits 4A of the above lighttransmitting substrate 4 are formed is the first information-recordingsurface 2, and the surface on which the pits 4B of the above transparentlayer 6 are formed is the second information-recording surface 3.

In addition, a first reflecting layer formed between the firstinformation surface 2 and the second information-recording surface 3 ismade of a material that has a certain degree of light transmittance, soas to permit light to be incident on the second information-recordingsurface 3 or to be reflected.

If three or more information-recording surfaces are to be formed, asecond transparent layer is formed on the second reflecting layer 7 onthe above transparent layer 6, and pits are formed on the secondtransparent layer to make a third information-recording surface, andfourth and subsequent information-recording surfaces can be similarlyformed.

A notable aspect of the above optical disk 1 is that a thermoplasticresin is chosen as the material for constructing the transparent layer6. This is because a thermoplastic resin sheet is used to manufacturethe optical disk 1 to be described later.

The thermoplastic resin sheet that constitutes the transparent layer 6is chosen, for a playback principle, from those that have a double passbirefringence of not more than ±50 nm, preferably not more than ±30 nm.This choice is made because as the double pass birefringence increasesbeyond ±50 nm, the resultant optical distortion will increasinglydegrade playback signals, thereby increasing error rates. Therefore, thematerial to be used as the thermoplastic resin sheet is preferably anacrylic resin (PMMA), polycarbonate resin(PC), or an amorphouspolyolefin resin.

A thermoplastic resin sheet equal to or less than 20 μm in filmthickness would reduce ease of handling and also tend to wrinkle duringmolding. Since the film thickness of the above transparent layer 6 isnearly equal to that of the thermoplastic resin sheet used, it would bedifficult for a light pickup of a playback device to track if thethermoplastic resin sheet were 100 μm or thicker. This, in turn, couldrequire a more complicated mechanism for the light pickup, making itundesirable to use any thermoplastic resin sheet film thicker than 100μm. Therefore, the film thickness of the thermoplastic sheet used shouldbe about 20 μm to about 100 μm, preferably about 25 μm to about 75 μm soas to result in transparent layer 6 with a film thickness of about 20 μmto about 100 μm.

Incident playback laser beams for reading the information recorded oneach of the information-recording surfaces thus constructed on theoptical disk 1 are directed from the underside of substrate 4. The lightpickup of a playback device can accurately recognize the spacing betweenthe adjacent information-recording surfaces, in which the spacing may beas narrow as several tens of microns (μm), and are focused onto thedesired information-recording surface, so as to allow reading out theinformation on each information-recording surface.

Since the information-recording surface (second information-recordingsurface 3) on a surface other than that of substrate 4 is formed ontransparent layer 6 which is constituted of a thermoplastic resin sheetwith a double birefringence within ±50 nm and a uniform film thicknessin the range of about 20 μm to about 100 μm, it is possible to producean optical disk in which the degradation of playback signals due to anoptical distortion is slight and the variation in spacing between theadjacent information-recording surfaces is very low.

That is, the variation in the film thickness of transparent layer 6 isabout equal to that of the thermoplastic resin, so that the use of athermoplastic resin sheet of uniform film thickness will allow thevariation in the spacing between the adjacent information-recordingsurfaces to fall within the allowable range. Therefore, thisconstruction will lead to a reduction in errors due to a variation inspacing between the adjacent information-recording surfaces and also toa reduction in requirements for the construction of a playback device.

EXAMPLE 1 OF A PROCESS FOR THE MANUFACTURE OF AN OPTICAL DISK

Next, the process for the manufacture of the above optical disk 1 isdescribed.

FIG. 4 is a drawing illustrating an embodiment of the process for themanufacture of the optical disk of this invention shown in FIG. 3. Forsimplicity, the description is made of a two-layered disk.

As illustrated in FIG. 4(A), an optical disk substrate 4 with pits 4A onthe surface thereof is molded, for example, by injection molding using astamper (not illustrated) with a pattern negative to pits A of the firstinformation-recording surface.

Next, as illustrated in FIG. 4(B), a first reflecting layer 5 is formedon the surface on which are formed pits 4A of substrate 4 by afilm-forming method such as sputtering, vacuum vapor depositing, spincoating, or the like. As described above, the first reflecting film is areflecting layer having a certain degree of light transmittance.

Then, a second information-recording surface is formed on the firstreflecting layer using a molding device 31 as illustrated in FIG. 5.FIG. 5 is used to explain the molding device 31.

FIG. 5 is a schematic drawing showing essential parts of the opticaldisk molding device shown in FIG. 3.

As shown in FIG. 5, the inside of a vacuum chamber 32 of the moldingdevice for the optical disk 1 is constructed in a way that allowsadjustment to a specific vacuum level by means of a vacuum pump 35.

Molds 33 and 34 are also positioned in the vacuum chamber 32. The mold33 is fixed in the vacuum chamber 32 and is used to mount substrate 4having the first information-recording surface 2 thereon. Onto the mold34 is mounted a stamper 36 to form the second information-recordingsurface 3. The mold 34 is mounted movably in the vertical direction inthe vacuum chamber 32 in such a way that it can move the mounted stamper36 while holding it horizontally until the stamper 36 comes into contactwith the substrate 4 mounted on the above mold 33.

In addition, thermoplastic resin sheet 37 which is used to generate atransparent layer 6 is unfolded and positioned between the molds 33 and34. As described above, the thermoplastic resin sheet 37, which isconstituted of a thermoplastic resin material having a double passbirefringence of not more than ±50 nm, has a uniform film thickness inthe range of about 20 μm to about 100 μm, is in continuous sheet formwider than the diameter of the above substrate 4, and is stored as awound roll, as illustrated by FIG. 5, within the vacuum chamber 32 (oroutside of the vacuum chamber 32). The wound thermoplastic resin sheet37 is unwound to a suitable length and unfolded to be held, via guiderollers 39A and 39B, between the mounted substrate 4 and the stamper 36.

Furthermore, the substrate 4 mounted on mold 33 and the stamper 36mounted on mold 34 are equipped with heaters, which are not illustrated,and the thermoplastic resin sheet 37 is equipped with a means forheating, not illustrated, such as an indirect infrared heater so thatthese can be heated to any designated temperatures. This setup is madeso as to melt the surface of the thermoplastic resin sheet 37 when thestamper 36 is pressed against the sheet as will be described later.

Thus, substrate 4, stamper 36, and thermoplastic resin sheet 37 are eachheated so that the thermoplastic resin sheet 37 should not come incontact with, but be apart from, the surfaces of the molds 33 and 34,the mounted substrate 4, and the stamper 36. It is preferred to keep thethermoplastic resin sheet under some tension in the lengthwise directionso as to prevent the thermoplastic resin sheet 37 between molds 33 and34 from slacking between guide rollers 39A and 39B.

Stopping the heating by the above heating means will allow thethermoplastic resin sheet 37 to cool and solidify, where some coolingmeans may be provided to facilitate cooling the thermoplastic resinsheet 37.

As illustrated in FIG. 4(C), the substrate 4 on which the firstreflecting layer is formed is mounted on mold 33, and the stamper 36having crenulated pits 36A formed with a pattern negative to pits 4B ofthe second information-recording surface 3 is mounted on mold 34. Asalso shown in FIG. 4(C), the substrate 4 is mounted with its surfacehaving the first reflecting layer 5 formed thereon (the firstinformation-recording surface 2) upward, and the stamper 36 is mountedwith its surface having a crenulated pattern 36 A formed thereondownward. As described above, thermoplastic resin sheet 37 is positionedbetween the mounted substrate 4 and the stamper 36.

Then, the inside of the vacuum 32 is brought to a specific level ofvacuum, and mold 34 is moved downward so as to interpose thethermoplastic resin sheet 37 between the substrate 4 and the stamper 36and to apply a specified pressure. Since the substrate 4, the stamper36, and the thermoplastic resin sheet 37 are each heated, the twosurfaces of the thermoplastic resin sheet 37 will melt. The two moltensurfaces allow pits 4B to be transferred to that surface ofthermoplastic resin sheet 37 in contact with the stamper and the othersurface thereof in contact with the substrate 4 to be adhered tosubstrate 4. After applied heat and pressure is held for a given periodof time, followed by suitably cooling to solidify the thermoplasticresin sheet 37, the stamper 36 is peeled off, and the thermoplasticresin sheet 37 is cut along the outer edge of substrate 4, as shown inFIG. 4 (D), resulting in the formation of a substrate having pits 4B ofthe second information-recording surface 3 formed on the surfacethereof.

The formation of the optical disk 1 is completed (FIG. 4(E)) by forminga film of aluminum, gold, or the like on the transparent layer 6 onwhich pits 4 have been formed by means of vacuum film formation such assputtering, vacuum vapor depositing, or the like, thereby forming asecond reflecting layer 7, by spin coating on the second reflectinglayer 7 with a film of a UV curable resin or the like to form aprotective layer 8, and then by printing a label (not illustrated) onthe protective layer 8.

The optical disk as manufactured as above will have the spacing betweenadjacent information-recording surfaces to be nearly identical to thethickness of the original thermoplastic resin sheet 37. Since thethermoplastic resin sheet 37 has a variation in film thickness of about±1 μm, the spacing between the adjacent information-recording surfacescan also be achieved at a similar magnitude of variation. Thisconstruction can eliminate a need for a rigorous control of the spacingbetween adjacent information-recording surfaces, which will lead toshortening of the time needed in the molding steps to form a secondinformation-recording surface 3 and will also allow manufacturing amultilayered optical disk with a very low variation in spacing betweenadjacent information-recording surfaces (the variation in film thicknessof transparent layer 6).

Although the above description of the process calls for molding thethermoplastic resin sheet 37 in a vacuum, it would obviously be possibleto mold without a vacuum. Nevertheless, molding the thermoplastic resinsheet 37 in a vacuum offers some advantages. Such an operation willallow the sheet 37 to be uniformly laminated without any chance oftrapping air bubbles at the moment when the sheet 37 and the stamper 36,and the substrate 4 and sheet 37, are joined together at the interfaces.Uneven lamination, if any, would result in film thickness irregularitiesfor the transparent layer 6, causing an out-of-focus problem, but sheetmolding in a vacuum can avoid such problems and provide a uniform lapeven for a large diameter disk.

EXAMPLE 2 FOR AN OPTICAL DISK

As shown in FIG. 6, both the first and second information-recordingsurfaces may be formed by molding thermoplastic resin sheet 37.

The first information-recording surface 2 of an optical disk 11 isformed in the same manner as that of the second information-recordingsurface of the above optical disk 1. That is, a mirror-surface substrate12 free of any crenulated features made by pits or the like is mountedon mold 33 of the above molding device; a stamper used to form pits 12Aof a first information-recording surface is mounted on mold 34 and ispressed with heat in the manner described above to transfer the pits12A, followed by cooling and cutting out the thermoplastic resin sheet37 into a first transparent layer 13. A first reflecting layer 5 isformed on the first transparent layer 13, followed by formingsequentially, as in the optical disk 1, a second transparent layer 14, asecond reflecting layer 7, and a protective film 8, thereby generatingoptical disk 11.

EXAMPLE 2 OF A PROCESS FOR THE MANUFACTURE OF AN OPTICAL DISK

A reflecting film may be precoated to one surface of the abovethermoplastic resin sheet in the above process for the manufacture ofoptical disks 1 and 11, which are shown in FIG. 3 and FIG. 6,respectively.

As illustrated in FIG. 7 (A), a reflecting film 38 is precoated to onesurface of the thermoplastic resin sheet 37, and the sheet is placed toposition the side having the reflecting film 38 formed thereon to facethe stamper 36. The reflecting film 38 coated to the thermoplastic resinsheet 37 becomes upon molding a second reflecting layer 7. When thethermoplastic resin sheet 37 having the reflecting layer coated theretois processed with heat and pressure as described above and rapidlycooled, the pattern of crenulated pits 36A of the stamper 36 istransferred to the surface of the thermoplastic resin sheet 37 havingthe reflecting film 38 formed thereon. Cutting out the moldedthermoplastic resin 37 along the outer periphery of substrate 4 resultsin the formation of a transparent layer 6 having the reflecting layercoated thereto, thereby forming a second information-recording surface3. Formation of a protective film 8 on the second reflecting layer 7 onthe transparent layer 6 gives optical disks 1 and 11.

If it is necessary to have different reflecting layers depending on theinformation-recording surfaces to be formed, thermoplastic resin sheetswith different reflecting films coated thereto may be prepared inseveral types so as to permit selection of the type appropriate to theinformation-recording surface to be formed.

Since the film of the reflecting layer is formed in a vacuum, it ispossible to integrate the film-forming device of the reflecting layersinto the vacuum chamber of the molding device 31. This will permitforming the reflecting layer film and moving to the molding step withoutremoving it out into the atmosphere in a one-pass equipment operation toconsolidate the film formation and molding and will improve theproductivity by an order of magnitude.

Optical disks in these examples of this invention are now described inmore detail using specific experimental examples as below:

Experiment Example 1

Two pieces of nickel stamper with digitized image signals inscribedthereinto in a 0.84 μm track pitch and a pit length 0.45 μm or longerwere used. One of them was used to produce a first information-recordingsurface by injection molding polycarbonate, thereby forming a substrate4 with a 120 mm diameter and a 1.2 mm thickness having pits 4A of thefirst information-recording surface formed thereon. A high transmittingfirst reflecting layer 5 was formed on the surface of the substrate 4having the formed pits.

Then, the substrate was mounted on mold 33 of the above molding device31 not equipped with either the above vacuum chamber 32 or vacuum pump35. The mold 33 was heated to 70° C. Acrylic resin (PMMA) sheet(thermoplastic sheet) 37 with a double pass birefringence in the planeof not more than ±15 nm was used, which was available as a 15 cm wideand 500 m length cut roll with a film thickness of 50 μm (film thicknessvariation range ±1 μm). This sheet was unfolded over the substrate 4.

Mold 34 with stamper 36 to be used to form a secondinformation-recording surface 3 was positioned to face substrate 4 inthe mold 33. Incidentally, the pit-formed surface of substrate 4, sheet37, the underside of stamper 36 were held parallel to, and apart from,each other.

The surface of the stamper 36 was heated to 85° C. and the sheet 37 inthe atmosphere was indirectly infrared-heated (not illustrated) to 75°C. Then, mold 34 was moved to press against mold 33 and was held for 5seconds to compress the sheet 37 interposed between the molds. The mold33 was then rapidly cooled to 50° C. and the mold 34 down to 60° C.; andthe sheet 37 was cut out along a 120 mm outer diameter by a circularblade (not illustrated) placed at an outer periphery of mold 33,followed by separating the molded substrate from the original sheet 37.Then, the two molds were opened to remove the substrate adhered to mold33. The completed substrate had pits 4B of the secondinformation-recording surface formed thereon in a 50 μm spacing apartfrom, and parallel to, the first information-recording surface 2. Thetotal molding time to form the transparent layer 6 to produce the secondinformation-recording surface was 30 seconds.

A second reflecting layer (aluminum) 7 was sputtered to a film thicknessof 70 nm onto the above transparent layer 6, followed by spin coatingthe layer with SD-11 (manufactured by Dainippon Ink & Chemicals, Inc.)to an 8 μm thick protective film 8, and the layer was then UV-cured.Lastly a label was printed over it to complete the manufacture of theoptical disk 1.

The optical disk 1 was evaluated for playback using a disk testerequipped with a laser wavelength of 635 nm and an objective lens with anumerical aperture, NA0.52. Signals from the two surfaces wereindividually observed by changing the focal position of the pickup, andit was found that eye patterns were cleanly open in the both layers topermit stable playback. Playback jitter was 15% for the first layer and12% for the second layer, indicating highly usable levels if operated inconjunction with an equalizer.

Experiment Example 2

This example used a rolled acrylic resin sheet (thermoplastic resinsheet) with one surface having a 70 nm film of a second reflecting layer(aluminum) 7 prcoated thereto (See FIG. 7(A)). This was used to replacethe acrylic sheet used in Example 1 and molded with the aluminizedsurface to come in contact with stamper 36. In this case, an operationsimilar to that of Example 1 was followed to fabricate a disk substrate,except that the inside of the vacuum chamber 32 of molding device 31 washeld at 1 torr. Since the molded transparent layer 6 in this experimentexample had already been aluminized, the formation of the secondreflecting layer 7 was omitted, and a protective film 8 was spin coatedover it to complete the manufacture of an optical disk 1.

A similar playback testing of this example also confirmed a stableplayback performance.

The optical disk in the above Experiment Example 1 may be sheet-moldedin a vacuum; similarly, the optical disk in the Experiment example 2 mayalso be sheet-molded without the use of a vacuum.

Although the above examples and Experiment examples were describedexclusively on read-only optical disks as illustrative examples, thisinvention is not limited to such applications, and can be applied toonce-writable and rewritable type disks as well as to optical cards andother optical information-recording media. When the application tooptical information-recording media involves a once-writable orrewritable type, the stamper 36 forms preformatting information such asguide grooves, and the like.

In addition, further applications may be made, such as the formation ofan antistatic layer on the underside of substrate 4 or the upper side ofprotective film 8 or printing of a label on the uppermost protectivefilm 8.

[Advantageous effect of the Invention]

-   (1) According to the optical disk of this invention, a multilayered    optical disk can be obtained with very little degradation of the    playback signal caused by optical distortions and with a very low    variation in spacing between adjacent information recording    surfaces. That is, since the variation in film thickness of the    transparent layer is about equal to that of the thermoplastic resin    sheet, use of a uniform thickness thermoplastic resin sheet permits    bringing the variation in spacing between adjacent    information-recording surfaces to an allowable range of error.

Therefore, this construction can reduce error rates caused by thevariations in spacing between adjacent information-recording surfacesand also to reduce the burden of requirements on the playback device tobe used.

-   (2) The optical disk manufacturing process of this invention    eliminates a rigorous control of the spacing between adjacent    information-recording surfaces in its manufacture and will allow    shortening of the time required to form an information-recording    surface in the molding steps. Since the variation in spacing between    adjacent information-recording surfaces is about equal to the    variation in film thickness of the original thermoplastic resin    sheet, the use of a uniform thickness thermoplastic resin sheet will    permit manufacture of a multilayered optical disk with a very low    variation in spacing between adjacent information-recording surfaces    (the variation in thickness of the transparent layer). That is, this    invention can improve both precision and productivity.

1. An optical information-recording medium that has at least two or more information-recording surfaces in the thickness direction of a light transmitting substrate and that allows reading of the information recorded on each information-recording surface of the two or more information-recording surfaces by varying the focal position of playback laser beam incident on and passing through said light transmitting substrate comprising: a transparent layer that is laminated onto said light transmitting substrate and is constituted of a thermoplastic resin sheet of uniform thickness; wherein an information-recording surface other than the surface of said light transmittins substrate is formed on said transparent layer.
 2. An optical information-recording medium as set forth in claim 1, wherein the thermoplastic resin sheet which constitutes said transparent layer is a thermoplastic resin sheet with a double-pass birefringence of not more than ±50 nm and a uniform film thickness in the range of about 20 μm to about 100 μm.
 3. An optical information-recording medium as set forth in claim 1, wherein the thermoplatic resin sheet which constitutes said transparent layer is an acrylic resin, polycarbonate resin, or amorphous polyolefin.
 4. A process for the manufacture of an optical information-recording medium that has at least two or more information-recording surfaces in the thickness direction of a light transmittin substrate and that allows reading of the information recorded on each information-recording surface of the two or more information-recording surfaces by varying the focal position of a playback laser beam incident on and passing through said light transmitting substrate comprising: a first step in which a thermoplastic resin sheet of uniform thickness is positioned between said light transmitting substrate and a stamper having a crenulated pattern formed thereon corresponding to pits or guide grooves; and a second step in which said light transmitting substrate and said stamper are heated, followed by pressing the light transmitting substrate and stamper against said thermoplastic sheet, thereby transferring the crenulated pattern formed on said stamper to the surface of said thermoplastic resin sheet and forming an information-recording surface on a surface other than that of said light transmitting substrate.
 5. A process for the manufacture of an optical information-recording medium as set forth in claim 4, wherein in said first step, a thermoplastic resin sheet with a light reflecting material coated to one of the surfaces is positioned with the surface to which the light reflecting material is coated to face said stamper side; and in said second stepm, an information-recording surface is formed on the surface of the sheet to which the light reflecting material is coated.
 6. A process for the manufacture of an opticl information-recording medium as set forth in claim 4, wherein the themoplastic resin used in said first step is thermoplastic resin sheet with a double pass birefringence of not more than ±50 nm and has a uniform film thicikness in the range of about 20 μm to about 100 μm.
 7. A process for the manufacture of an optical information-recording medium as set forth in claim 4, wherein the thermoplastic resin used in said first step is an acrylilc resin, polycarbonate resin, or amorphous polyolefin. 